Orbital implant

ABSTRACT

An orbital implant includes a body of bioactive material having macropores of at least 400 μm, and a cap of bioactive material having substantially no pores or only micropores smaller than 50 μm. The cap covers a portion of the body.

THIS INVENTION relates to an orbital implant.

According to the invention, there is provided an orbital implant whichincludes a body of bioactive material having macropores of at least 400μm, and a cap of bioactive material having substantially no pores oronly micropores smaller than 50 μm, with the cap covering a portion ofthe body.

The term ‘bioactive material’ used in this specification has its usualgenerally accepted meaning or definition, namely that it is ‘a materialthat elicits a specific biological response at the interface of thematerial which results in the formation of a bond between the tissuesand the material’, as provided by L. L. Hench and J. Wilson in “AnIntroduction to Bioceramics”, Advanced Series in Ceramics—Vol. 1, Ed. L.L. Hench and J. Wilson, World Scientific, Singapore, N.J., London, HongKong (1993) p. 7.

The bioactive material may be a calcium phosphate material or compoundsuch as a hydroxyapatite; a bioactive glass, which can typically bebased on SiO₂, Na₂O, CaO and/or P₂O₅; a bioactive glass ceramic, whichcan be similar in composition to bioactive glass but which incorporatesadditionally MgO, CaF₂ and/or metal oxides; or a composite materialCONFIRMATION COPY comprising a polymer containing bioactive materialparticles, such as particles of a calcium phosphate compound, abioactive glass and/or a bioactive glass ceramic.

The orbital implant may preferably be spherical. It will thus be of asize so that it can be inserted into, and fit into, the orbit of amammal, either to replace the contents of an eye following eviscerationor to replace the eyeball following enucleation. Thus, when it is to beimplanted into the orbit of an adult human, it may have a diameter ofabout 20 mm.

The macropores of the body may be substantially spherical so that theyhave diameters of said at least 400 μm. Preferably, the diameters of themacropores do not exceed 1000 μm.

Some macropores may be in communication with the outer surface of thebody. In other words, when such macropores are present, the body willhave irregularly spaced surface indentations or dimples. Adjacentmacropores in the body may be interconnected by openings and/orpassageways. Thus, by means of the macropores which are in communicationwith the body outer surface and the openings and/or passageways betweenadjacent macropores, open paths to the body outer surface, defined bythe macropores, are provided in the implant body. The interconnectingopenings or passageways between adjacent macropores may have diametersgreater than 50 μm, preferably greater than 100 μm.

In other words, the body may contain substantially no isolated or closedmacropores.

The macropores in the body may occupy from 40% to 85% by volume,preferably about 60% by volume, of the body.

The body may also have micropores smaller than 50 μm. At least some ofthese micropores may be of irregular shape. Thus, they may be in theform of interstitial spaces, for example, interstitial spaces betweenparticles of bioactive material, resulting from incomplete sintering ofthe particles during formation of the body. The sizes of thesemicropores are then dependent on the sizes of the bioactive materialparticles from which the body is sintered. However, these microporeswill have a maximum dimension smaller than 50 μm, and their maximumdimension may typically be of the order of 1 μm, or even smaller.Instead, or additionally, at least some of the micropores may be ofregular shape, eg substantially spherical so that their diameters arethus smaller than 50 μm. The micropores, when present, may occupy from3% to 70% by volume, preferably about 40% by volume, of themacropore-free bioactive material, ie the residual bioactive materialaround the macropores.

All the spherical micropores present in the body may be of substantiallythe same size, while all the irregularly shaped micropores may be ofsubstantially the same size. The irregularly shaped micropores may besmaller than the spherical micropores. For example, when the irregularlyshaped micropores are of the order of 1 μm or smaller, the sphericalmicropores may have diameters of at least 10 μm, and may typically havediameters of 10-45 μm.

Adjacent micropores in the body are then preferably interconnected byopenings and/or passageways. Some micropores may also be interconnectedto the macropores by openings and/or passageways. The micropores willthus, by means of these openings and/or passageways, provide open pathsto the macropores, as well as, together with the macropores, open pathsto the outer surface of the body. In other words, there may thus besubstantially no isolated or closed micropores in the body.

More specifically, both interstitial micropores and spherical microporesmay be present in the body, with adjacent spherical micropores beinginterconnected by interstitial micropores which thus constitute theinterconnecting openings or passageways. The interstitial microporeswill then also interconnect the spherical micropores to the macropores.

The body may thus have a trimodal pore size distribution, comprisingmacropores, which may be in the size range 400-1000 μm; largermicropores which may be in the size range smaller than 50 μm but atleast 10 μm; and smaller micropores which are 1 μm or smaller.

The cap may, in one embodiment of the invention, be of bioactivematerial containing substantially no pores. However, in anotherembodiment of the invention, the cap may contain pores; however, thepores will then be micropores smaller than 50 μm, ie the pores will thenbe irregular micropores and/or spherical micropores, as hereinbeforedescribed. In other words, the cap is then characterized thereby that itcontains no pores larger than 50 μm. Thus, it will contain no macroporesas hereinbefore described.

The cap, which is thus an anterior cap, may be in the form of a circularconcave or dish-shaped disc integrated with or embedded in the body ofbioactive material. The diameter of the rim of the cap may be the sameas the diameter of the implant; however, preferably, it has a smallerdiameter than that of the implant. Preferably, the diameter of the rimof the cap may be about three-quarters that of the implant.

The cap will thus be thin relative to the diameter of the implant. Thus,its thickness may be no more than half the diameter of the implant, andpreferably about one-fortieth of the diameter of the implant.

While the bioactive material of the cap can, at least in principle, bedifferent to that of the body, it is envisaged that the body and the capwill normally be of the same bioactive material. The bioactive materialmay, in particular, be synthetic hydroxyapatite.

The orbital implant of the invention is thus, in use, placed into anorbit of a mammal.

The placing of an orbital implant of the integrated type, ie an orbitalimplant which, in use, becomes integrated through tissue ingrowth andvascularization, such as that of the invention, following eviscerationor enucleation, is known.

The mammal will thus be one who has had an ocular enucleation orevisceration, or who needs an implant replacement. Use of the orbitalimplant according to the invention will, it is believed, result infibrovascular tissue ingrowth into the entire body of the implant, withthe comparatively smooth cap resulting in little or no erosion ofanterior tissue, including the conjunctiva, taking place.

After the implant has been placed into the orbit, eye muscles aretypically attached to the implant, whereafter the implant is coveredwith tissue including conjunctiva, and a period of healing allowedduring which fibrovascular tissue ingrowth into the implant occurs.Thereafter, an artificial eye or prosthesis can be fitted over theconjunctiva, adjacent the cap of the implant. It follows thus that whenthe implant is placed into the orbit, it is orientated such that the capfaces anterior tissue including the conjunctiva.

The invention will now be described in more detail by way of example andwith reference to the accompanying diagrammatic drawings.

In the drawings,

FIG. 1 shows a front view of an orbital implant according to oneembodiment of the invention;

FIG. 2 shows a side view of the orbital implant of FIG. 1;

FIG. 3 shows an enlarged cross-sectional view of part of the orbitalimplant of FIG. 1;

FIG. 4 shows an enlarged cross-sectional view, similar to that of FIG.3, of an orbital implant according to another embodiment of theinvention; and

FIG. 5 shows a portion of the cross-sectional view of FIG. 4, enlargedeven further.

Referring to FIGS. 1 to 3, reference numeral 10 generally indicates anorbital implant according to one embodiment of the invention.

The implant 10 is substantially spherical, and has a diameter of about20 mm. It includes a body 12 of synthetic hydroxyapatite havingspherical macropores 14 as well as spherical micropores 16. Themacropores 14 are all of substantially the same size, and have diametersof 400-1000 μm, typically about 800 μm. The macropores 14 occupy about60 vol % of the body 12. Some of the macropores 14 are in communicationwith the outer surface 15 of the body 12, as can be seen in FIG. 3. Itwill be appreciated that at least some adjacent macropores may beinterconnected (not shown) by openings or passageways (not shown).

The micropores 16 are also all of substantially the same size, and havediameters less than 50 μm, eg about 10-45 μm. The micropores 16 occupyabout 40 vol % of the residual hydroxyapatite, ie the hydroxyapatitematerial between the macropores 14. The body 12 is thus solid save forthe macropores and micropores therein.

The implant 10 also includes a thin anterior cap 18 of hydroxyapatitematerial having no macropores. The cap 18 thus contains either no poresat all or only micropores (not shown) having maximum dimensions lessthan 50 μm, eg having maximum dimensions of about 1 μm. When present,the micropores will occupy about 40% by volume of the cap material. Thecap 18 is thus characterized thereby that it contains no pores largerthan 50 μm.

The cap 18 is in the form of a concave dish, and the rim 20 of the cap18 has a diameter of about three-quarters that of the implant 10. Thus,when the implant 10 has a diameter of about 20 mm, the rim 20 of the cap18 has a diameter of about 15 mm.

The thickness of the cap 18 is about one-fortieth the diameter of theimplant 10. Thus, for an implant 10 having a 20 mm diameter, thethickness of the cap 18 will be about 0.5 mm.

The cap 18 thus covers only a portion of the body 12.

Referring to FIGS. 4 and 5, reference numeral 100 generally refers to anorbital implant according to another embodiment of the invention.

Parts of the implant 100 which are the same or similar to those of theorbital implant 10, are indicated with the same reference numerals.

The implant 100 is also substantially spherical (not shown), and has abody 12 and an anterior cap (not shown) as hereinbefore described inrespect of the implant 10. The body 12 of the implant 100 also hasspherical macropores 14; however, apart from some of the macropores 14of the implant 100 being in communication with the outer surface of thebody 12 of the implant 100 (as hereinbefore described in respect of theimplant 10) adjacent macropores 14 are interconnected by openings 102.The diameters of the openings 102 are typically about 100 μm or greater.The implant 100 is normally manufactured by a sintering process such asthat hereinafter described, and the interconnection of adjacentmacropores then typically arises as a result of adjacent macroporescoalescing together during the sintering process. As a result of thecommon openings 102 between adjacent macropores 14 and the macropores 14which are in communication with the outer surface of the implant body,open paths to the body outer surface are defined by the macropores inthe body 12. Thus, the body 12 contains substantially no closed orisolated macropores.

The body 12 of the implant 100 also contains spherical micropores 16(see FIG. 5), as hereinbefore described in respect of the implant 10.Moreover, it also contains irregular micropores 104 in the form ofinterstitial spaces between hydroxyapatite particles 106, resulting fromincomplete sintering of hydroxyapatite particles 106 during formation ofthe body 12 by means of a sintering process such as that hereinafterdescribed. Although the hydroxyapatite particles are shown, in FIG. 5,as distinct separate particles, this is for ease of illustration only.In fact, adjacent particles will thus be partially sintered together sothat such adjacent particles can no longer be viewed as being distinctparticles (as shown in FIG. 5) but rather merge so that they are in theform of an agglomerated mass containing the spherical macropores 14, thespherical micropores 16 and the irregular micropores 104. The sizes ofthe micropores 104 are substantially the same, and are dictated by thesizes of the hydroxyapatite particles 106 used for sintering. Thus, whenthe particle sizes are about 1 μm, the maximum dimensions of themicropores 104 may be about 1 μm, or smaller.

Adjacent micropores 16 and 104 are thus interconnected. Typically,adjacent micropores 16 are interconnected by micropores 104.Additionally, the micropores 104 and/or the micropores 16 are alsointerconnected to the macropores 14. Thus, the micropores 16, 104together with the macropores 14, also define open paths to the outersurface of the implant body 12. There are thus substantially no closedor isolated micropores 16, 104 in the implant body.

The irregular micropores 104 typically occupy about 40% by volume of theresidual hydroxyapatite, ie the macropore free hydroxyapatite, while thespherical micropores 16 typically occupy about 10% by volume of theresidual hydroxyapatite.

To manufacture the implant 100, a mixture A is prepared by compoundinghydroxyapatite powder having a mean particle size of about 1 μm, with apolymeric binder of a type suitable for injection moulding or extrusion;grinding the mixture to less than 300 μm particle size; and mixingstearic acid balls with a size distribution between 500-1000 μmtherewith.

A mixture B is prepared by compounding hydroxyapatite powder having amean particle size of about 1 μm with the same polymeric binder as usedfor mixture A; and grinding the mixture to less than 300 μm particlesize.

The mixture A is loaded into a die suitable for pressing of a sphere.This die includes a piston which will create a depression on the surfaceof the sphere during pressing, with the depression having the size andshape of the desired cap 18. The mixture A is lightly pressed to form asphere containing the said depression. The depression is then filledwith a correct amount of the mixture B. Thereafter the structureincluding the sphere with powder is consolidated by pressing to form aspherical compact comprising mixture A with an intimately bound cap ofmixture B. The structure is sintered at a temperature below 1100° C.

It will be appreciated that when an implant in accordance with theinvention is made by means of a sintering process such as thathereinbefore described, interstitial micropores 104 which result fromincomplete sintering of adjacent hydroxyapatite particles, will thusnormally be present. Thus, such interstitial micropores will also bepresent in the body 12 of the implant 10 when it is manufactured bymeans of such a sintering process.

The implants 10, 100 can be implanted into the orbit or eye socket of ahuman who has had an ocular enucleation or evisceration, or who needs animplant replacement. The implants can be placed according to knownprocedures for integrated implants. For example, in the case of anevisceration, the implant is implanted to replace the eye contents. Or,in the case of enucleation, the implant is placed without covering orwith a covering or wrapping of tissue or artificial material into theeye muscle cone (not shown), and the eye muscles attached directly tothe implant 10, 100 or to the implant wrapping. Instead, the eye musclescan be wrapped around the implant 10, 100 and secured together withoutdirect attachment of the eye muscles to the implant 10, 100. Theanterior surface of the implant is covered with tissue including theconjunctive. The cap 18 faces the conjunctiva. A healing period is thenallowed. During this healing period, fibrovascular tissue ingrowth intothe entire body 12 is promoted by the bioactive hydroxyapatite surfacesin conjunction with the open paths provided by the macropores 14, themicropores 16 and the micropores 104. Following this period of healing,the implant is integrated and, due to the muscle attachment, capable ofmovement. Thereafter the prosthesis, ie an artificial eye, is located inposition adjacent the cap 18, to obtain an artificial eye with naturalappearance and good motility.

It is believed that the orbital implant of this invention addresses twocommon causes of complications associated with the use of orbitalimplants of the integrated type. These are incomplete fibrovasculartissue ingrowth into the implant interior and erosion of anterior tissueby rough surface protrusions of a porous body.

The orbital implant of this invention addresses the first of thesecauses of complications by promoting complete ingrowth of fibrovasculartissue into the implant interior. This is achieved by the implant of theinvention having three modified material properties, as compared toproperties commonly encountered in known porous orbital implants:

-   -   Firstly, the macropore size is substantially increased, by a        factor of 2 to 5, over that commonly encountered in known porous        orbital implants. Macropore size is generally restricted in        porous orbital implants, to achieve improved mechanical        properties and an even external roundness. This is particularly        important when the implant is made from materials derived from        natural sources such as coral or processed bovine bone, where        the external shape has to be achieved by machining. With such        materials, the external roundness can be extremely uneven due to        the fracture of brittle protrusions and pore edges during        machining. It also produces undesirable sharp fracture surfaces.        In the orbital implant of this invention, an even roundness is        readily achieved due to the entirely synthetic manufacture        thereof, which eliminates any need for machining of a brittle        surface and thereby avoids protrusions with sharp fracture        surfaces.    -   Secondly, this larger macropore size is associated with a        corresponding increase in the size of the interconnecting        openings between adjacent macropores, to the extent that even        the interconnecting openings are larger than the macropores        commonly encountered in known porous orbital implants.    -   Thirdly, the orbital implant of the invention can have an        engineered distribution of open micropores along the macropore        surfaces and in the bulk of the ceramic material. These        micropores are present in a very high volume fraction, typically        40 vol % of the macropore-free hydroxyapatite material. This        engineered micropore distribution distinguishes the orbital        implant of this invention over known bioceramic orbital implant        materials, where microporosity is either absent in the material        source or regarded as detrimental to mechanical strength and        therefore eliminated to a large extent. The small micropore size        present at high volume fraction serves to significantly increase        surface roughness at the cellular level. It further achieves an        increase in surface area, up to a factor of 70, over that of an        equivalent material without the micropore distribution. This is        desirable in that it increases the bioactivity of the ceramic        material. It further achieves a strong associated capillary        force exerted by the ceramic bulk, which is absent from        materials without the high level of microporosity since the        force is proportional to the volume fraction of micropores and        inversely proportional to the micropore size. The high degree of        surface roughness, the large surface area and the strong        capillary force result in immediate and strong adhesion of        tissue to the material, avoiding motion and, importantly,        micromotion of tissue against the implant. It further results in        rapid ingress and retention of fluid with improved cell        attachment. When combined with the inherent bioactive property        of the hydroxyapatite composition it is further associated with        direct tissue apposition, that is direct tissue ongrowth without        intervening fibrous tissue as in the case of polymer materials.        Finally, it is believed that the combined material properties        may be associated with binding and expression of autologous        growth factors at the site, which promote early tissue healing.

Thus, to summarize, the large macropore size combined with largeinterconnecting opening size result in open access for fluid and tissueingrowth to the central regions of the implant. Along the innermacropore surfaces and bulk of the material, the material has beenengineered to exhibit high surface roughness, high surface area, strongcapillary force and inherent bioactivity. This ensures immediate strongtissue attachment, elimination of micromotion, rapid ingress andretention of fluid with improved cell attachment, direct tissueapposition without intervening fibrous tissue.

The orbital implant of this invention further addresses the second causeof complications associated with porous orbital implants of theintegrated type. This is the tendency of porous materials to present arough surface with sharp protrusions to anterior tissue, leading toerosion of the tissue and complications such as exposure of the implant.By incorporating a cap of comparative smoothness, the implant does notpresent sharp edges to anterior tissue. This serves to avoid erosion ofthe anterior tissue. The cap is an integral part of the implantstructure and is comprised of the same material as the implant body,incorporating the same micropore distribution as described. Hence itexhibits a similar degree of tissue attachment, capillary force and highbioactivity as the porous ceramic body, even in the absence ofmacropores, since the inherent high bioactivity of the microporoushydroxyapatite allows direct tissue attachment even in the absence ofsignificant tissue ingrowth. From a tissue engineering and materialspoint of view, it is significant and advantageous that a seamlesstransition is achieved from porous body to cap, particularly in such asensitive location where the overlying anterior tissue is relativelythin. This fully incorporated cap serves to further distinguish thematerial from known orbital implants. Thus, it is different from a capof different material over the anterior region, which will introduce anartificial transition from a tissue engineering and materials point ofview, since two different materials are unlikely to evoke identicalresponse and achieve a seamless transition. It is also different from apolymer cap, in that a polymer cap will exhibit low or no bioactivityand will require some different means to achieve attachment of theanterior tissue. It is also different from a temporary resorbablecoating over the implant, such as a polymer- or inorganic cement-basedcap, since a resorbable coating will merely delay ingrowth and ongrowthto the ceramic while the underlying roughness of the porous body willeventually present again. Finally, it is extremely difficult orimpossible from a ceramic processing point of view to attach andincorporate such a cap on pre-densified material, such as acoral-derived or bone-derived material.

Thus, the implant body with incorporated cap does not present sharp andrough edges to the anterior tissue, thereby avoiding erosion of theanterior tissue. Further, full incorporation of the cap is advantageousin that it presents a seamless transition from porous body to cap from atissue engineering and materials point of view. Further, the capmaterial exhibits high surface area, suitable roughness at the cellularlevel only, strong capillary force and inherent high bioactivity. Theseproperties jointly promote tissue attachment, elimination ofmicromotion, rapid ingress and retention of fluid with improved cellattachment, direct tissue apposition without intervening fibrous tissue.

1. An orbital implant which includes a body of bioactive material havingmacropores of at least 400 μm, and a cap of bioactive material havingsubstantially no pores or only micropores smaller than 50 μm, with thecap covering a portion of the body.
 2. An implant according to claim 1,which is substantially spherical.
 3. An implant according to claim 2,which has a diameter of about 20 mm.
 4. An implant according to claim 1,wherein the macropores in the body are substantially spherical so thatthey have diameters of at least 400 μm, and, optionally, wherein thediameters of the macropores do not exceed 1000 μm.
 5. An implantaccording to claim 1, wherein some macropores are in communication withthe outer surface of the body and wherein adjacent macropores in thebody are interconnected by openings and/or passageways, so that openpaths to the outer surface of the body are thereby provided in the body.6. An implant according to claim 5, wherein substantially no isolated orclosed macropores are present in the body.
 7. An implant according toclaim 5, wherein the openings and/or passageways which interconnectadjacent macorpores have diameters greater than 50 μm.
 8. An implantaccording to claim 5, wherein the macropores in the body occupy from 40%to 85% by volume of the body.
 9. An implant according to claim 5,wherein the body has micropores smaller than 50 μm.
 10. An implantaccording to claim 9, wherein at least some of the micropores are ofirregular shape, and have a maximum dimension smaller than 50 μm.
 11. Animplant according to claim 9, wherein at least some of the microporesare substantially spherical so that their diameters are thus smallerthan 50 μm.
 12. An implant according to claim 9, wherein adjacentmicropores in the body are interconnected by openings and/or passagewaysand wherein some micropores are also interconnected to the macropores byopenings and/or passageways so that the micropores, by means of thoseopenings and/or passages, provide open paths to the macropores.
 13. Animplant according to claim 12, wherein substantially no isolated orclosed micropores are present in the body.
 14. An implant according toclaim 9, wherein some of the micropores are of irregular shape and arein the form of interstitial spaces between incompletely sinteredbioactive material particles, and wherein some of the micropores are ofsubstantially spherical shape, with irregular micropores interconnectingadjacent spherical micropores, and also connecting spherical microporesto macropores.
 15. An implant according to claim 14, wherein all thespherical micropores are of substantially the same size while all theirregular micropores are of substantially the same size, with theirregular micropores being smaller than the spherical micropores.
 16. Animplant according to claim 14, wherein substantially no isolated orclosed micropores are present in the body.
 17. An implant according toclaim 9, wherein the micropores occupy from 3% to 70% by volume of themacropore-free bioactive material.
 18. An implant according to claim 1,wherein the cap is in the form of a circular concave disc integratedwith the body of bioactive material.
 19. An implant according to claim1, which is substantially spherical, and wherein the cap has a thicknesswhich is no more than half the diameter of the implant.
 20. An implantaccording to claim 1, wherein the bioactive material of the body andthat of the cap are the same, and is hydroxyapatite.